1. Field of the Invention
This invention generally relates to security systems, specifically to security systems which employ video equipment for motion detection. Disclosed is a system which reduces the number of false alarms generated by video motion detector systems in response to video image changes which are not related to motion.
2. Discussion of the Related Art
Video systems are well known in the field of security systems. In a typical security system, one or more video cameras are placed so as to provide a field of view of the area under surveillance. These video cameras convert a visual image into an electronic form suitable for transmission. A control station, either co-located within the surveillance area or remote from the area, receives the signals from these cameras and displays the video image at a console, for security assessment and recording. Typically, a person monitors the images from the cameras on a video screen and initiates security measures if the received image indicates unauthorized activities. Often the monitoring person (hereinafter the monitor) is responsible for monitoring the images from multiple cameras simultaneously, and means are provided to assist in this process.
Automated motion detection systems are employed to alert the monitor of the presence of activity within the view of a camera, as typified in U.S. Pat. No. 4,458,266. These motion detection systems operate by detecting changes in the sequential electronic images of the same scene. A change in the scene implies the entry or exit of an item from that scene. When a change is detected, an alarm is sent to the monitor for a security assessment. The monitor will view the sequence of images which caused the alarm, as well as other images, from this camera or others, to determine whether the alarm requires the initiation of security measures such as notifying the police or activating a warning signal. These motion detection systems can be co-located with the camera, or remote from the camera. They are often co-located with the camera and operate so as to transmit the images to the control station only in the event of an alarm, thereby saving communications bandwidth and costs.
Environmental changes will cause the video image to change; for example, in an outside environment, the video image at sunset will be different from the video image at noon. Because motion detectors operate by comparing video images for changes, and environmental changes create such changes, means must be provided to avoid the generation of an alarm signal in response to environmental changes. Conventionally, noting that most environmental changes are slowly changing phenomenon, motion detection systems avoid the generation of alarms in response to environmental changes by comparing images which occur relatively closely spaced in time. That is, for example, instead of comparing the image at noon with an image at sunrise, the image at noon is compared to the image at a fraction of a second before noon. A person or object entering the scene will introduce a noticable change between images of a fraction of a second diffence in time, but the change of environment in that same fraction of time will be insufficient to trigger an alarm. In the conventional system, the compared image is continually updated, to maintain the fraction of time difference between images. That is, following the aforementioned comparison between the noon image and the noon-minus-a-fraction image, the noon-plus-a-fraction image is compared to the noon image, and so on.
This sequential compare and update process results in motion detection systems which are sensitive to relatively rapid changes to the scene, and are insensitive to relatively slow changes to the scene, as desired. Because they are sensitive to rapid changes in scenes, conventional motion detectors are sensitive to rapid environmental changes as well. A lightning bolt at night will cause a significant change to sequential video images, and will cause the motion detector associated with these images to generate an alarm, obviating their effectiveness during a lightning storm. The headlights of a car, illuminating the area within a camera's field of view, will also trigger erroneous alarms, which often limits the choice of placement or field of view of a security camera.
Security systems often also include a means for masking a portion of the image area from motion detection. Such systems allow movement within the masked areas, and sound an alarm for movement in other areas, both areas within the field of view of the camera. An interior scene may, for example, comprise a walkway adjacent to a secure area. Even though movement in the walkway can be masked to prevent alarms being generated in response to such movement, the turning on or turning off of the lighting for the walkway will cause the secure area image to change, resulting in a false alarm.